Kathy, this is a common problem amongst all the machine brands, people are always asking this same question on the various groups I'm on.
But you're correct, it would be nice if the machine gave a message to indicate the issue.
Want to know a funny thing? If you do a combined design (JEF+) in Horizon Link that completely fills the hoop and then open that same JEF design in MBX software and try to send it to the usb, the program will insist that it's too large for the hoop! Even though MBX was made to Janome specs and the hoop reads as the same size in both programs.